Multiple bandwidth phase lock filters for multimode radios

ABSTRACT

An apparatus and method for filtering a signal in a phase lock loop is disclosed. An typical apparatus for filtering a phase error signal comprises a first filter subcircuit receiving a phase error signal from a phase comparator and filtering the error signal when enabled, a second filter subcircuit receiving the phase error signal from the phase comparator and filtering the error signal when enabled and first and second enable switches which are activated in combination to control filtering of at the error signal. The first filter subcircuit is enabled by activating a first combination of the enable switches and the second filter subcircuit is enabled by activating a second combination of the enable switches. A fast acquisition subcircuit can also be used to temporarily underdampen the filter and improve overall performance of the phase lock loop circuit. The filter can be used to obtain multimode operation in a cellular radio.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to systems and methods forimplementing phase lock loop (PLL) circuits in multimode radios, andparticularly for implementing PLL circuits in multimode code divisionmultiple access (CDMA) cellular radios that include global positioningsystem (GPS) receivers and other global systems for mobilecommunications (GSM) technologies.

[0003] 2. Description of the Related Art

[0004] Phase Locked Loop (PLL) circuits are well known and used forfrequency control in a variety of applications. For example, they can beconfigured as frequency multipliers, demodulators, tracking generatorsor clock recovery circuits. Each of these applications may demanddifferent characteristics, however they all use the same basic circuitconcept.

[0005]FIG. 1A illustrates a block diagram of a conventional PLL circuit100. The operation of this circuit 100 is typical of all phase lockedloops. It is basically a feedback control system that controls the phaseof a voltage controlled oscillator (VCO) 108. The reference input signal102 is applied to one input of a phase comparator 104. The other inputof the phase comparator 104 is connected to the output of the VCO 108via the feedback loop 110. The output of the phase comparator 104 is avoltage proportional to the phase difference between the two inputs.This signal is applied to the loop filter 106. The characteristics ofthe loop filter 106 can be selected to achieve the desired closed loopdynamic characteristics of the PLL circuit 100. The filtered signalcontrols the VCO 108 and the output of the VCO 108 is directed back tothe phase comparator 104 via the feedback loop 110 to “lock” the outputto the reference input signal 102.

[0006] Normally the loop filter 106 is designed to match thecharacteristics required by the application of the PLL circuit 100. Ifthe PLL circuit 100 is to acquire and track a varying reference signal102, the bandwidth of the loop filter 106 will be greater than if it isapplied to a more limited input frequency. The frequency range which thePLL circuit 100 will accept and lock on is known as the capture range.Once the PLL circuit 100 is locked and tracking a signal, the range offrequencies that the PLL circuit 100 will follow is called the trackingrange. Generally, the tracking range is larger than the capture range.

[0007] The characteristics of the loop filter 106 also determine howquickly the frequency of the reference signal 102 can change and stillmaintain lock. This is termed as the maximum slewing rate. The narrowerthe loop filter 106 bandwidth is, the slower the response and smallerthe capture range will be.

[0008] PLL circuits are particularly useful in communicationselectronics for generating accurate and stable oscillator referencesignals in radio receivers and transmitters. For example, the PLLcircuit output can be used for channel selection for such receivers andtransmitters.

[0009] However, evolving and diversifying communications standards andparticularly those related to cellular radios, have resulted in a needfor radio transceivers which can operate in different modes andaccommodate more than one standard and frequency band. For example, acellular radio which supports CDMA 1× and/or global system for mobilecommunications (GSM) general packet radio service (GPRS) and widebandcode division multiple access (WCDMA) is desirable. To support multimodecapabilities, there is a need for more than one reference oscillatorwithin a single transceiver. A conventional design would include twoseparate PLL circuits and switch between them as necessary, however suchan approach increases the size, cost and overall design efficiency ofthe transceiver. In addition, there is also a need for such PLL circuitsto quickly acquire the new frequency when they are switched from oneband to another.

[0010] Accordingly, there is a need in the art for PLL circuits whichoperate across multiple frequencies to support one or more VCOs andmultiple standards in multimode radio cellular devices. For example,standards such as CDMA 1×, GPS and/or GSM/GPRS and WCDMA should all besupportable by such circuits. There is also a need for such circuits tooperate at very high speed, quickly acquiring a new frequency when theyare switched to a different mode. Moreover, there is a need in the artto obtain all of the foregoing, while maximizing the design efficiencyand minimizing size and costs. The present invention meets all theseneeds.

SUMMARY OF THE INVENTION

[0011] An apparatus and method for filtering a signal in a phase lockloop is disclosed. An typical apparatus for filtering a phase errorsignal comprises a first filter subcircuit receiving a phase errorsignal from a phase comparator and filtering the error signal whenenabled, a second filter subcircuit receiving the phase error signalfrom the phase comparator and filtering the error signal when enabledand first and second enable switches which are activated in combinationto control filtering of the error signal. The first filter subcircuit isenabled by activating a first combination of the enable switches and thesecond filter subcircuit is enabled by activating a second combinationof the enable switches. A fast acquisition (FAQ) subcircuit can also beused to temporarily underdampen the filter and improve overallperformance of the phase lock loop circuit. The filter can be used toobtain multimode operation in a cellular radio.

[0012] In one embodiment, where the second filter subcircuit has ahigher bandwidth than the first filter subcircuit, the second filtersubcircuit can be momentarily activated to fast acquisition with thefirst filter subcircuit.

[0013] In another embodiment, the first filter subcircuit and the secondfilter subcircuit share common circuit elements. In addition, when thefirst enable switch is activated, either filter subcircuit can bealternately referenced to a ground or a common cathode voltageselectable by a reference switch. Futhermore, the first and secondenable switches can be field effect transistors. The filter can be usedto obtain multimode operation in a cellular radio.

[0014] In one embodiment, a fast acquisition subcircuit including a fastacquisition enable switch and wherein fast acquisition of one of thefirst and second filter subcircuits is enabled when the fast acquisitionswitch is enabled. The fast acquisition subcircuit can be temporarilyenabled to underdampen the filter, thereby improving the overallperformance of the PLL circuit. The fast acquisition subcircuit cancomprise a fast acquisition resistor which is used to alter theresistance characteristic of the filter subcircuit when the fastacquisition switch is enabled. Furthermore, the fast acquisitionsubcircuit can be coupled to the first or second filter subcircuit. Aswith the filter subcircuits, the fast acquisition subcircuit can bealternately referenced to a ground and common cathode voltage selectableby a reference switch.

[0015] In another shared element embodiment of the invention, the sharedcommon circuit elements are a first capacitor and a first resistor inseries both connected in parallel to a second capacitor. Furthermore,the first filter subcircuit can be a subset of the second filtersubcircuit.

[0016] In different embodiments of the invention various switchcombinations may be used to operate the filter and change itscharacteristics. In one embodiment the first combination comprisesactivating the first enable switch and deactivating the second enableswitch which places a first capacitor in series connection with a firstresistor both in parallel connection with second capacitor. The secondcombination comprises deactivating the first enable switch andactivating the second enable switch which places a first and thirdcapacitor in parallel connection, both in series connection with a firstresistor and the first and third capacitor and first resistor are allplaced in parallel connection with a second and fourth capacitor inseries connection. In another embodiment, the first combinationcomprises activating the both the first enable switch and the secondenable switch which places a first capacitor in series connection with afirst resistor both in parallel connection with second capacitor. Thesecond combination comprises deactivating both the first enable switchand the second enable switch which places a first and third capacitorand a first resistor all in series connection, all placed in parallelconnection with a second and fourth capacitor in series connection.

[0017] In another embodiment, the first and second filter subcircuitseach comprise a resistor-capacitor filter including a first capacitorand a first resistor in series both connected in parallel to a secondcapacitor.

[0018] Finally, the first and second filter subcircuits of the presentinvention can be implemented on a single integrated circuit. The firstand second enable switches can be internal or external to the integratedcircuit.

[0019] In addition the present invention also teaches a method havingfeatures like those of the described apparatus. The typical methodcomprises the steps of filtering a signal in a phase lock loop,comprising the steps of receiving a phase error signal from a phasecomparator, filtering the error signal with a first filter subcircuitwhen a first enable switch is activated and filtering the error signalwith a second filter subcircuit when a second enable switch isactivated.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Referring now to the drawings in which like reference numbersrepresent corresponding parts throughout:

[0021]FIG. 1A illustrates a block diagram of a conventional PLL circuit;

[0022]FIG. 1B illustrates a block diagram of a PLL circuit employing aloop filter embodiment of the present invention;

[0023]FIG. 2A is a circuit diagram of a first embodiment of the presentinvention;

[0024]FIG. 2B is a circuit diagram of a second embodiment of the presentinvention;

[0025]FIG. 3A is a circuit diagram of a third embodiment of the presentinvention;

[0026]FIG. 3B is a table showing the enable switch combinations of thethird embodiment;

[0027]FIG. 4A is a circuit diagram of a fourth embodiment of the presentinvention;

[0028]FIG. 4B is a table showing the enable switch combinations of thefourth embodiment;

[0029]FIG. 4C is a circuit diagram of a modified fourth embodiment ofthe present invention;

[0030]FIG. 4D is a table showing the enable switch combinations of themodified fourth embodiment; and

[0031]FIG. 5 is a flowchart illustrating a method of filtering a phaseerror signal according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0032] In the following description of the preferred embodiment,reference is made to the accompanying drawings which form a part hereof,and in which is shown by way of illustration a specific embodiment inwhich the invention may be practiced. It is to be understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from the scope of the present invention.

[0033] 1. Overview

[0034] The present invention is directed to a PLL circuit design capableof supporting more than one mode of operation. Such a circuit is usefulin cellular radios designed to operate with more than one bandwidth andcommunication standard. For example, a PLL circuit which can supportCDMA 1× or GSM (GPRS) and WCDMA would enable a cellular radio to operatein either system as required by the location or system reference of theuser. Thus, such a PLL circuit enable system roaming of the user.

[0035]FIG. 1B is a block diagram of a PLL circuit 112 embodiment usingthe present invention. A reference input signal 114 is supplied to thecircuit 112. The input can be optionally directed through a divider 116to reduce the frequency of the signal 114. The signal 114 is thendirected to a phase comparator 118 which determines a phase differencebetween the signal 114 and an output from a voltage controlledoscillator 124 downstream from the phase comparator 118. The output fromthe voltage controlled oscillator 124 is fed back to the phasecomparator via feedback loop 126. Optionally, the feedback loop 126 mayinclude a loop divider 128 to reduce the frequency of the output beforeprocessing by the phase comparator 118. The phase comparator 118produces an output based upon the phase difference between the signal114 and the output from the voltage controlled oscillator 124. The phasecomparator 118 may be digitally controlled and include a charge pump toproduce an output comprising regulated current pulses as the output ofthe phase comparator 118.

[0036] The output of the phase comparator 118 is used to control theoutput frequency of the VCO 124. To achieve this, regulated currentpulses from the phase comparator 118 must be converted into a voltagevia the loop filter which can be applied to the VCO 124. In addition,the filtered phase difference signal may be supplied to more than oneVCO 124.

[0037] In one embodiment, this can be accomplished by a filter 120. Thefilter 120 receives the output of the phase comparator 118 andappropriately filters the signal for use by the VCO 124. In support ofmultiple modes and bandwidths, different filtering properties arerequired. The filtering properties of the filter are alterable throughone or more switches enabled with signals applied at inputs 122. Thus,the output of the filter 120 is applied to the one or more VCOs 124 tolock the phase of the output to the reference input signal 114.

[0038] 2. Example Filter Embodiments

[0039] The present invention is directed to a PLL filter wherein filtersubcircuits are selectively enabled to alter the properties of thefilter. Various embodiments of the filter may be used in the PLL circuitof FIG. 1B to support multimode operation and multiple bandwidths in acellular radio.

[0040]FIG. 2A is a circuit diagram of a first filter 200 embodiment ofthe present invention. The filter 200 receives an input from the phasecomparator 118 (or particularly, the charge pump of the comparator 118)and then filters the input to produce an output voltage to one or moreVCOs 124. The filter 200 comprises at least two filter subcircuits 202A,202B which are separately enabled by switches 204A, 204B, respectively.To enable filter subcircuit 202A, first switch 204A is activated andsecond switch 204B is deactivated in a first switch combination.Likewise, to activate filter subcircuit 202B, second switch 204B isactivated and first switch 204A is deactivated in a second switchcombination. The subcircuits 202A, 202B each include aresistor-capacitor filter with a first resistor 206A, 206B and firstcapacitor 208A, 208B in series, both in parallel with a second capacitor210A, 210B. The filter subcircuits 202A, 202B are selectively activatedby inputs 212A, 212B (such as the inputs 122 of FIG. 1B) applied totheir respective switches 204A, 204B. In one embodiment, the switches204A, 204B are semiconductor switches such as field effect transistors(FETs), however, other switch elements can be used, includingmicroelectromechanical switches, npn or pnp transistors, pin diodes, orsimilar devices.

[0041] Each of the filter subcircuits 202A, 202B are separately designedto operate with the remainder of the PLL circuit 112 to lock to adifferent frequency and thereby support a different mode of operation.For example, first filter subcircuit 202A may be optimized for CDMA 1×mode operation, while the second filter subcircuit 202B is optimized forGSM GPRS mode operation. Design of the filter subcircuits 202A, 202B iseffected through the selection of values for the resistor 206 andcapacitor 208, 210 circuit elements. Any number of filter subcircuits202 may be added to the filter in the manner described to support anynumber of bands with one or more VCOs in the overall PLL design.

[0042] Each loop filter (e.g., filter subcircuit) can be designed withindependent charge pump current (CPi) settings. When switching betweenmultiple filter subcircuits, such as filter subcircuits 202A, 202B, thechange in the filter bandwidth may require a related change in thesupplied charge pump current supplied by the phase comparator 118.

[0043]FIG. 2B is a circuit diagram of a second filter 250 embodiment ofthe present invention. The filter 250 also includes filter subcircuits202A, 202B, however, this filter 250 is shown with a slightly differentarrangement than that of FIG. 2A. Preferably, the filter 250 can beimplemented with switches 204A, 204B on an integrated circuit usingdiscrete external filter components. This filter subcircuits 202A, 202Bor this second filter 250 are enabled with switch combinations identicalto the first filter 200 embodiment.

[0044] In addition, at least one of the subcircuits 202B includes a fastacquisition subcircuit 214. The fast acquisition subcircuit 214 includesa fast acquisition resistor 216 which is placed in parallel with theresistor 206B of one of the filter subcircuits 202B when fastacquisition switch 218 is enabled. In fast acquisition, the totalresistance of the filter decreases and the charge pump current increasesby a factor, F, and causes a bandwidth expansion. Fast acquisition maybe temporarily activated to enable fast acquisition of the signal. Orfast acquisition can be forced to obtain a wider loop bandwidth. Thecharge pump current changes by a factor, F, equal to the square of theratio of bandwidth change. For example, if switching the filtersubcircuit results in doubling the bandwidth, the charge pump currentincreases four times. In contrast, if switching the filter subcircuitresults in halving the bandwidth, the charge pump current is cut to onequarter the value.

[0045] Table 1 below provides examples of bandwidth expansion, FAQ andthe total effective resistance (R_(TOTAL)) for parallel fast acquisitionresistor 216, which are correlated by a factor (F). As discussed above,the charge pump current changes by the factor, F, when the bandwidthchanges. Closing fast acquisition switch 218 expands the bandwidth offilter subcircuit 202B by the factor, F, (i.e. properly setting thecharge pump current). Thus, three selectable filters are effectivelyavailable in the circuit 250, filter subcircuits 202A and 202B andfilter subcircuit 202B with the fast acquisition subcircuit 214 enabled.TABLE 1 Fast Acquisition Bandwidth Expansion for Parallel Fastacquisition Resistor Bandwidth Fast Acquisition Expansion (FAQ)R_(TOTAL) = (R1//R1′) $\approx \sqrt{F}$

×F $\frac{R1}{\sqrt{F}}$

$\approx \sqrt{4}$

×4 $\frac{R1}{\sqrt{4}}$

$\approx \sqrt{8}$

×8 $\frac{R1}{\sqrt{8}}$

$\approx \sqrt{16}$

×16  $\frac{R1}{\sqrt{16}}$

[0046] The total effective resistance for parallel resistors 206B, 216is given by the following equation (1). $\begin{matrix}{R_{TOTAL} = \frac{{R1}^{\prime} \times {R1}}{{R1}^{\prime} + {R1}}} & (1)\end{matrix}$

[0047] Thus, the necessary value for the fast acquisition resistor (R1′)216 can be determined by applying the appropriate R_(TOTAL) value to thefollowing equation (2). $\begin{matrix}{{R1}^{\prime} = \frac{{R1} \times R_{TOTAL}}{{R1} - R_{TOTAL}}} & (2)\end{matrix}$

[0048] Or, in the general form, including the factor, F, R1′ can bedetermined from equation (3). $\begin{matrix}{{R1}^{\prime} = \frac{R1}{\sqrt{F} - 1}} & (3)\end{matrix}$

[0049] For example, to obtain a bandwidth expansion of {squareroot}{square root over (16)} and FAQ of ×16 (under a commensuratesixteen fold charge pump current increase), the fast acquisitionresistor 216 should have a value of approximately one-third the resistor206B of the associated filter subcircuit 202B. The switch 218 can beenabled with an input 212C applied at the switch 218. With fastacquisition enabled, the loop bandwidth of the filter subcircuit 202B isincreased by underdamping the filter, thereby providing a quicker tuningtime as indicated in the table above. Although shown only on a singlefilter subcircuit 202B, fast acquisition may be implemented on any orall of the subcircuits of the filter, thereby increasing the effectivenumber of available filters. After tuning, fast acquisition may bedisabled to return the filter to its regular damping characteristics asdetermined by the filter subcircuit(s) 202A, 202B (i.e. the effectivetotal resistance is returned to R1). Also, when leaving fastacquisition, the charge pump current returns to its previous steadystate current. Such temporary enabling of the fast acquisitionsubcircuit 214 can be used to improve the overall tuning time of the PLLcircuit.

[0050] It should also be noted that in some embodiments fast acquisitioncan be implemented by temporarily activating a filter subcircuit havinga higher bandwidth. For example, if filter subcircuit 202B has a higherbandwidth than filter subcircuit 202A, filter subcircuit 202B can beapplied to fast acquisition filter subcircuit 202A. In this case, filtersubcircuit 202B is momentarily enabled to produce a similar effect, asdiscussed above, as applying a dedicated fast acquisition subcircuit.The charge pump current is temporarily adjusted for the bandwidths ofthe two filter subcircuits 202A and 202B. In this way, fast acquisitioncan also be effected for at least the lower bandwidth subcircuits infilter circuits without a fast acquisition subcircuit (such as filtercircuit 200).

[0051] Furthermore, FIG. 2B also illustrates that the filter subcircuits202A, 202B and fast acquisition subcircuit 214 may be enabled byreferencing the subcircuits 202, 214 to ground or a common cathodevoltage (VCC) through their respective switches 204, 218. Selection ofeither ground or VCC for the filter subcircuit 202B or the fastacquisition subcircuit 214 is enabled through the respective referenceswitches 220B, 220C. (Note there is no reference switch 220A for filtersubcircuit 202A in the example shown.) This arrangement is useful in theevent that the PLL circuit 112 uses more than one VCO 124 havingdifferent varactor references. Although the reference switches 220B,220C are shown schematically as mechanical switches 220B, 220C, otherswitch elements can be used, including semiconductor switches, e.g.,FETs, npn or pnp transistors, pin diodes, microelectromechanicalswitches, or similar devices.

[0052]FIGS. 3A and 3B are a circuit and control matrix of a third filtercircuit 300 embodiment of the present invention. FIG. 3A is diagram ofthe filter circuit 300. In this filter 300, multimode support isintegrated into a single integrated circuit 316 operated by switches.Such an integrated design is accomplished by employing a common resistorvalue in the otherwise separate subcircuits 202A, 202B of the previousembodiments. The resulting circuit includes subcircuits 302A, 302B whichuse common filter elements for different modes. In the example shown,the filter circuit elements of subcircuit 302A are a subset of thefilter circuit elements of subcircuit 302B. In other words, the commonfilter circuit elements are all the filter circuit elements ofsubcircuit 302A. Different modes of operation are effected by enablingand disabling switches 304A, 304B and fast acquisition enable switch 218according to the switch combinations of table of FIG. 3B to produce thecircuit characteristics shown.

[0053] In a first mode, filter subcircuit 302A is enabled by activatingfirst filter switch 304A and deactivating second filter switch 304B.This produces a filter with a first capacitor 306 in series with a firstresistor 308 and both in parallel with a second capacitor 310. In asecond mode, filter subcircuit 302B is enabled by activating secondfilter switch 304B and deactivating first filter switch 304A. Also, inthis second mode a charge pump current setting different from that ofthe first mode may be used. In this case, a third capacitor 312 isplaced into the second filter subcircuit 302B in parallel with the firstcapacitor 306 and thus provides an effective total capacitance equal tothe sum of the first and third capacitors 306, 312, as described by thefollowing equation (4).

C _(TOTAL) =C1+C1′  (4)

[0054] This combination of capacitors 312, 306 is in series connectionwith the first resistor 308. In addition, the second capacitor 310 isnow placed in series combination with a fourth capacitor 314, yielding atotal effective capacitance given by the following equation (5).$\begin{matrix}{C_{TOTAL} = \frac{{C2} \times {C2}^{\prime}}{{C2} + {C2}^{\prime}}} & (5)\end{matrix}$

[0055] The effective total capacitances and resistances are shown in thetable of FIG. 3B according to the indicated combined operation ofswitches 304A, 304B, 218.

[0056] As with the previous embodiments, a fast acquisition subcircuit214 is also appended to the filter circuit 300 and operates according tothe characteristics previously described. In this case, because thesubcircuits 302A, 302B are conjoined, the fast acquisition subcircuit214 may be activated for either subcircuit 302A, 302B. Alternately, thefilter subcircuit (e.g., 302B) having a higher bandwidth can betemporarily activated to fast acquisition the lower bandwidth subcircuit(e.g., 302A) as previously discussed with respect to filter circuits 200and 250. The filter circuit 300 also includes a reference switch 220 forselection of either ground or VCC for the filter subcircuit 302.

[0057] The third filter circuit 300 shown produces a filter in which thefirst mode of operation (enabling subcircuit 302A) has a lower bandwidththan the second mode of operation (enabling subcircuit 302B). This ismade possible by using a common first resistor 308 for entire filtercircuit 300. As a result, the circuit is more compact and thus all thefilter switches and fast acquisition switch may be integrated into thePLL integrated circuit 316.

[0058]FIGS. 4A and 4B are a circuit 400 and control matrix for a fourthembodiment of the invention. FIG. 4A is a circuit diagram of filtercircuit 400. This filter embodiment is designed with a first filtersubcircuit 402A and a second filter subcircuit 402B merged in a mannersimilar to the third filter embodiment above. The first filtersubcircuit 402A and the second filter subcircuit 402B share commonfilter circuit elements.

[0059]FIG. 4B is a table showing the switch combinations of the fourthfilter 400 embodiment. In this filter circuit 400, selection of thefirst or second filter subcircuit 402A, 402B is not effected byactivating one switch and deactivating another switch as in the previousembodiments. To enable the first filter subcircuit 402A, the first andsecond enable switches 404A, 404B are both activated in a first switchcombination. To enable the second filter subcircuit 402B, the first andsecond enable switches 404A, 404B are both deactivated in a secondswitch combination. Here also, the first and second filter subcircuits402A, 402B can use different charge pump current settings. In addition,a fast acquisition switch 406 is used to enable a fast acquisitionsubcircuit 408A for the filter in a manner similar to the previousembodiments.

[0060] Fast acquisition with the fast acquisition subcircuit 408A inthis filter circuit 400 is only available when the second enable switch404B is deactivated. This is because the second enable switch 404Bshorts the fast acquisition subcircuit 408A when it is activated.Because the switch combinations determine that the second enable switch404B is deactivated only when the second filter subcircuit 402B isenabled, fast acquisition is only available when the second filtersubcircuit 402B is enabled in this example. In addition, when the fastacquisition subcircuit 408A is enabled, a fast acquisition resistor 410is placed in parallel with only one of two resistors 412 or 414. FIG. 4Ashows the fast acquisition resistor 410 connected in parallel toresistor 414. Therefore, for some designs the fast acquisitionresistance can be eliminated so that enabling fast acquisition merelyentails shorting the resistor 414. In other words, the fast acquisitionresistor 410 would have a value of zero. In this case, the totaleffective resistance becomes just the value of the resistor 412. In anycase, the charge pump current must be appropriately adjusted by thefactor, F, to keep the loop stable when fast acquisition is enabled.

[0061] However, fast acquisition can still be effected for filtersubcircuit 402A by momentarily switching to a higher bandwidth filtersubcircuit 402B as previously discussed regarding filter circuits 250and 300. The charge pump current must be temporarily adjusted to thecharge pump current setting of filter subcircuit 402B. Thus, the locktime performance can be improved even without a dedicated fastacquisition subcircuit.

[0062] Because two series connected resistors 412, 414 are used in thesecond filter subcircuit 402B, the fast acquisition bandwidth expansionis somewhat different from the previous embodiments. In this case, therequired R_(TOTAL) is based upon the sum of the two resistors 412, 414as shown in Table 2 below. Fast acquisition modifies only one resistancevalue of the series connected resistors (i.e., R1″). TABLE 2 FastAcquisition Bandwidth Expansion for Fast acquisition Resistor inParallel with One of Multiple Series-Connected Resistors Bandwidth FastAcquisition Expansion (FAQ) R_(TOTAL) = R1 + R1′//R1″ $\approx \sqrt{F}$

×F $\frac{{R1} + {R1}^{''}}{\sqrt{F}}$

$\approx \sqrt{4}$

×4 $\frac{{R1} + {R1}^{''}}{\sqrt{4}}$

$\approx \sqrt{8}$

×8 $\frac{{R1} + {R1}^{''}}{\sqrt{8}}$

$\approx \sqrt{16}$

×16  $\frac{{R1} + {R1}^{''}}{\sqrt{16}}$

[0063] Thus, the appropriate fast acquisition resistor 410 value (R1′)is somewhat different than in the previous embodiments. The necessarytotal resistance can be determined from the following equation (6).$\begin{matrix}{R_{TOTAL} = {{R1} + \frac{{R1}^{\prime} \times {R1}^{''}}{{R1}^{\prime} + {R1}^{''}}}} & (6)\end{matrix}$

[0064] Solving for the fast acquisition resistor value yields thefollowing equation (7). $\begin{matrix}{{R1}^{\prime} = \frac{{R1}^{''}\left( {R_{TOTAL} - {R1}} \right)}{{R1}^{''} + {R1} - R_{TOTAL}}} & (7)\end{matrix}$

[0065] As before, the general solution can be obtained by applying thegeneral form of R_(TOTAL) including fast acquisition factor, F, to yieldequation (8). $\begin{matrix}{{R1}^{\prime} = \frac{{R1}^{''}\left\lbrack {R^{''} - {{R1}\left( {\sqrt{F} - 1} \right)}} \right\rbrack}{\left( {{R1}^{''} + {R1}} \right)\left( {\sqrt{F} - 1} \right)}} & (8)\end{matrix}$

[0066] The fourth filter 400 embodiment has a first filter subcircuit402A with characteristics like those of the previous embodiments, afirst capacitor 416 and first resistor 412 in a series, both in parallelwith a second capacitor 418. However, the second filter subcircuit 402Bdoes not use only the first resistor 412 in this embodiment. A secondresistor 414 and a third capacitor 420 are also placed in seriesconnection with the first resistor 412 and first capacitor 416 when thesecond filter subcircuit 402B is enabled. As described in the previousembodiments, this yields a total effective resistance of the sum of theresistors 412, 414 and a total effective capacitance of the reciprocalof the sum of the reciprocals of the capacitors 416, 420. A fourthcapacitor 422 is also placed in series with the second capacitor 418 inthis second filter subcircuit 402B. This also yields a total effectivecapacitance of the reciprocal of the sum of the reciprocals of thecapacitors 418, 422 for this leg of the circuit. The total effectivevalues for each of the circuit elements are provided in the table ofFIG. 4B. Values for these various elements are selected such that thefirst filter subcircuit 402A obtains a low bandwidth, while the secondfilter subcircuit obtains a high bandwidth.

[0067]FIGS. 4C and 4D are a circuit 450 and control matrix for amodified embodiment of the circuit 400 of FIGS. 4A and 4B. In thiscircuit 450, the fast acquisition resistor 410 of the fast acquisitionsubcircuit 408B is tapped in at a point between the resistor 412 and thecapacitor 416. The circuit 450 is otherwise identical to that of FIG.4A. Consequently, fast acquisition operates somewhat differently,however, the circuit 450 otherwise functions in the same manner as theprevious circuit 400.

[0068] As shown in FIG. 4D, fast acquisition is now available when thefirst filter subcircuit 402A is activated. Because enabling fastacquisition connects just two resistors in parallel, the first resistor412 and the fast acquisition resistor 410, fast acquisition operates ina manner very similar to that of the second and third embodimentspreviously described.

[0069] When fast acquisition is activated with the second filtersubcircuit 402B in the modified circuit 450, the circuit functions verysimilar to the second and third embodiments. However, in this case, thesecond resistor 414 is now added to the first resistor 412 and both areconnected in parallel with the fast acquisition resistor 410. Theanalysis is identical, except that the first resistor R1 is now replacedwith the sum of R1 and R1′. Thus, the Table 3 below provides examples ofbandwidth expansion, FAQ and the total effective resistance (R_(TOTAL)),which are correlated by a factor, F (i.e. the charge pump currentmultiplier). TABLE 3 Fast Acquisition Bandwidth Expansion for Fastacquisition Resistor in Parallel with Multiple Series-ConnectedResistors Bandwidth Fast Acquisition Expansion (FAQ) R_(TOTAL) = [(R1 +R1″)//R1′] $\approx \sqrt{F}$

×F $\frac{{R1} + {R1}^{''}}{\sqrt{F}}$

$\approx \sqrt{4}$

×4 $\frac{{R1} + {R1}^{''}}{\sqrt{4}}$

$\approx \sqrt{8}$

×8 $\frac{{R1} + {R1}^{''}}{\sqrt{8}}$

$\approx \sqrt{16}$

×16  $\frac{{R1} + {R1}^{''}}{\sqrt{16}}$

[0070] The total effective resistance for the combination of resistors412, 414 and 406 is given by the following equation (9). $\begin{matrix}{R_{TOTAL} = \frac{{R1}^{\prime} \times \left( {{R1} + {R1}^{''}} \right)}{{R1} + {R1}^{\prime} + {R1}^{''}}} & (9)\end{matrix}$

[0071] Thus, solving equation (9), the necessary value for the fastacquisition resistor (R1′) 410 can be determined by applying theappropriate R_(TOTAL) value to the following equation (10).$\begin{matrix}{{R1}^{\prime} = \frac{\left( {{R1} + {R1}^{''}} \right) \times R_{TOTAL}}{\left( {{R1} + {R1}^{''}} \right) - R_{TOTAL}}} & (10)\end{matrix}$

[0072] In a general form, including the factor, F, R1′ can now bedetermined from equation (11). $\begin{matrix}{{R1}^{\prime} = \frac{{R1} + {R1}^{''}}{\sqrt{F} - 1}} & (11)\end{matrix}$

[0073] In effect, the addition of the second resistor (R1″) 414 in themodified circuit 450 is equivalent to increasing the value of the firstresistor 412.

[0074] 3. Example Method Embodiment

[0075]FIG. 5 is a flowchart 500 illustrating a method of filtering aphase error signal according to the invention. The method begins byreceiving a phase error signal from a phase comparator at block 502. Thephase error signal is filtered with a first filter subcircuit when afirst enable switch is activated at block 504. The phase error signal isfiltered with a second filter subcircuit when a second enable switch isactivated at block 506. Optionally, the error signal may be fastacquisitiond by a fast acquisition subcircuit while at least one of thefirst and second filter subcircuits are filtering the error signal whena third enable switch is activated at block 508. Such fast acquisitionmay be temporarily enabled to improve the overall performance of the PLLcircuit.

CONCLUSION

[0076] This concludes the description including the preferredembodiments of the present invention. The foregoing description of thepreferred embodiment of the invention has been presented for thepurposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed. Manymodifications and variations are possible in light of the aboveteaching.

[0077] It is intended that the scope of the invention be limited not bythis detailed description, but rather by the claims appended hereto. Theabove specification, examples and data provide a complete description ofthe manufacture and use of the apparatus and method of the invention.Since many embodiments of the invention can be made without departingfrom the scope of the invention, the invention resides in the claimshereinafter appended.

What is claimed is:
 1. An apparatus for filtering a phase error signal,comprising: a first filter subcircuit receiving a phase error signalfrom a phase comparator and filtering the error signal with the firstfilter subcircuit when enabled; a second filter subcircuit receiving thephase error signal from the phase comparator and filtering the errorsignal with the second filter subcircuit when enabled; and a firstenable switch and a second enable switch which are activated incombination to control filtering of the error signal; wherein the firstfilter subcircuit is enabled by activating a first combination of thefirst and second enable switches and the second filter subcircuit isenabled by activating a second combination of the first and secondenable switches.
 2. The apparatus of claim 1, wherein the second filtersubcircuit has a higher bandwidth than the first filter subcircuit andthe second filter subcircuit is momentarily activated to fastacquisition with the first filter subcircuit.
 3. The apparatus of claim1, further comprising a fast acquisition subcircuit including a fastacquisition enable switch and wherein fast acquisition of one of thefirst and second filter subcircuits is enabled when the fast acquisitionswitch is activated.
 4. The apparatus of claim 3, wherein the fastacquisition subcircuit is temporarily enabled to underdampen thefiltering.
 5. The apparatus of claim 3, wherein the fast acquisitionsubcircuit comprises a fast acquisition resistor which is placed inparallel connection with a first resistor of one of the first and secondfilter subcircuits when the fast acquisition switch is activated.
 6. Theapparatus of claim 5, wherein the fast acquisition resistor comprises aresistance of approximately R1′ determined by${{R1}^{\prime} = \frac{R1}{\sqrt{F} - 1}},$

where R1 comprises the first resistor of the one of the first and secondfilter subcircuits and F comprises a fast acquisition factor.
 7. Theapparatus of claim 5, wherein the one of the first and second filtersubcircuits further comprises a second resistor in series connectionwith the first resistor.
 8. The apparatus of claim 7, wherein the fastacquisition resistor comprises a resistance of approximately R1′determined by${{R1}^{\prime} = \frac{{R1}^{\prime\prime}\left\lbrack {R^{\prime\prime} - {{R1}\left( {\sqrt{F} - 1} \right)}} \right\rbrack}{\left( {{R1}^{\prime\prime} + {R1}} \right)\left( {\sqrt{F\quad} - 1} \right)}},$

where R1 comprises the first resistor and R1″ comprises the secondresistor of the one of the first and second filter subcircuits and Fcomprises a fast acquisition factor.
 9. The apparatus of claim 7,wherein the fast acquisition resistor comprises a resistance ofapproximately R1′ determined by${{R1}^{\prime} = \frac{{R1} + {R1}^{\prime\prime}}{\sqrt{F} - 1}},$

where R1 comprises the first resistor and R1″ comprises the secondresistor of the one of the first and second filter subcircuits and Fcomprises a fast acquisition factor.
 10. The apparatus of claim 1,wherein the first filter subcircuit and the second filter subcircuitshare common filter circuit elements.
 11. The apparatus of claim 10,wherein the shared common filter circuit elements are a first capacitorand a first resistor in series both connected in parallel to a secondcapacitor.
 12. The apparatus of claim 10, wherein the first filtersubcircuit comprises filter circuit elements that are a subset of filtercircuit elements of the second filter subcircuit.
 13. The apparatus ofclaim 10, wherein the first combination comprises activating the firstenable switch and deactivating the second enable switch to place a firstcapacitor in series connection with a first resistor and both the firstcapacitor and first resistor in parallel connection with secondcapacitor.
 14. The apparatus of claim 10, wherein the second combinationcomprises deactivating the first enable switch and activating the secondenable switch which places a first and a third capacitor in parallelconnection and the first and third capacitor both in series connectionwith a first resistor and the first and third capacitor and the firstresistor are all placed in parallel connection with a second and afourth capacitor in series connection.
 15. The apparatus of claim 10,wherein the first combination comprises activating both the first enableswitch and the second enable switch to place a first capacitor in seriesconnection with a first resistor and both the first capacitor and firstresistor in parallel connection with a second capacitor.
 16. Theapparatus of claim 10, wherein the second combination comprisesdeactivating both the first enable switch and the second enable switchto place a first and a third capacitor and a first resistor all inseries connection and all in parallel connection with a second and afourth capacitor in series connection.
 17. The apparatus of claim 1,wherein the first and the second enable switches are field effecttransistors.
 18. The apparatus of claim 1, wherein when the first enableswitch is activated the first filter subcircuit is alternatelyreferenced to a ground and a common cathode voltage selectable by areference switch.
 19. The apparatus of claim 1, wherein when the secondenable switch is activated the second filter subcircuit is alternatelyreferenced to a ground and a common cathode voltage selectable by areference switch.
 20. The apparatus of claim 1, wherein the first andsecond filter subcircuits each comprise a resistor-capacitor filterincluding a first capacitor and a first resistor in series bothconnected in parallel to a second capacitor.
 21. The apparatus of claim1, wherein the first and second filter subcircuits are implemented on asingle integrated circuit.
 22. The apparatus of claim 21, wherein thefirst and second enable switches are external to the integrated circuit.23. The apparatus of claim 21, wherein the first and second enableswitches are internal to the integrated circuit.
 24. An apparatus forfiltering a phase error signal, comprising: a first filter subcircuitmeans for receiving a phase error signal from a phase comparator andfiltering the error signal with the first filter subcircuit whenenabled; a second filter subcircuit means for receiving the phase errorsignal from the phase comparator and filtering the error signal with thesecond filter subcircuit when enabled; and a first enable switchingmeans and a second enable switching means for activating in combinationto control filtering of the error signal; wherein the first filtersubcircuit is enabled by activating a first combination of the first andsecond enable switches and the second filter subcircuit is enabled byactivating a second combination of the first and second enable switches.25. The apparatus of claim 24, wherein the second filter subcircuitmeans has a higher bandwidth than the first filter subcircuit means andthe second filter subcircuit means is momentarily activated to fastacquisition with the first filter subcircuit means.
 26. The apparatus ofclaim 24, further comprising a fast acquisition subcircuit means forfast acquisition one of the first and second filter subcircuits when afast acquisition switching means for enabling the fast acquisitionsubcircuit means is activated.
 27. The apparatus of claim 26, whereinthe fast acquisition subcircuit means is temporarily enabled tounderdampen the filtering.
 28. The apparatus of claim 26, wherein thefast acquisition subcircuit means comprises a fast acquisition resistorwhich is placed in parallel connection with a first resistor of one ofthe first and second filter subcircuit means when the fast acquisitionswitch is activated.
 29. The apparatus of claim 28, wherein the fastacquisition resistor comprises a resistance of approximately R1′determined by ${{R1}^{\prime} = \frac{R1}{\sqrt{F} - 1}},$

where R1 comprises the first resistor of the one of the first and secondfilter subcircuit means and F comprises a fast acquisition factor. 30.The apparatus of claim 28, wherein the one of the first and secondfilter subcircuit means further comprises a second resistor in seriesconnection with the first resistor.
 31. The apparatus of claim 30,wherein the fast acquisition resistor comprises a resistance ofapproximately R1′ determined by${{R1}^{\prime} = \frac{{R1}^{\prime\prime}\left\lbrack {R^{\prime\prime} - {{R1}\left( {\sqrt{F} - 1} \right)}} \right\rbrack}{\left( {{R1}^{\prime\prime} + {R1}} \right)\left( {\sqrt{F\quad} - 1} \right)}},$

where R1 comprises the first resistor and R1″ comprises the secondresistor of the one of the first and second filter subcircuit means andF comprises a fast acquisition factor.
 32. The apparatus of claim 30,wherein the fast acquisition resistor comprises a resistance ofapproximately R1′ determined by${{R1}^{\prime} = \frac{{R1} + {R1}^{\prime\prime}}{\sqrt{F} - 1}},$

where R1 comprises the first resistor and R1″ comprises the secondresistor of the one of the first and second filter subcircuit means andF comprises a fast acquisition factor.
 33. The apparatus of claim 24,wherein the first filter subcircuit means and the second filtersubcircuit means share common filter circuit elements.
 34. The apparatusof claim 33, wherein the shared common filter circuit elements are afirst capacitor and a first resistor connected in series both the firstcapacitor and first resistor connected in parallel to a secondcapacitor.
 35. The apparatus of claim 33, wherein the first filtersubcircuit means comprises filter circuit elements that are a subset offilter circuit elements of the second filter subcircuit means.
 36. Theapparatus of claim 33, wherein the first combination comprisesactivating the first enable switching means and deactivating the secondenable switching means which places a first capacitor in seriesconnection with a first resistor both in parallel connection with secondcapacitor.
 37. The apparatus of claim 33, wherein the second combinationcomprises deactivating the first enable switching means and activatingthe second enable switching means which places a first and thirdcapacitor in parallel connection, both in series connection with a firstresistor and the first and third capacitor and first resistor are allplaced in parallel connection with a second and fourth capacitor inseries connection.
 38. The apparatus of claim 33, wherein the firstcombination comprises activating the both the first enable switchingmeans and the second enable switching means which places a firstcapacitor in series connection with a first resistor both in parallelconnection with second capacitor.
 39. The apparatus of claim 33, whereinthe second combination comprises deactivating both the first enableswitching means and the second enable switching means which places afirst and third capacitor and a first resistor all in series connection,all placed in parallel connection with a second and fourth capacitor inseries connection.
 40. The apparatus of claim 24, wherein the first andsecond enable switching means are field effect transistors.
 41. Theapparatus of claim 24, wherein when the first enable switching means isactivated, the first filter subcircuit means is alternately referencedto a ground and a common cathode voltage selectable by a referenceswitching means.
 42. The apparatus of claim 24, wherein when the secondenable switching means is activated the second filter subcircuit meansis alternately referenced to a ground and a common cathode voltageselectable by a reference switching means.
 43. The apparatus of claim24, wherein the first and second filter subcircuit means each comprise aresistor-capacitor filter including a first capacitor and a firstresistor in series both connected in parallel to a second capacitor. 44.The apparatus of claim 24, wherein the first and second filtersubcircuit means are implemented on a single integrated circuit.
 45. Theapparatus of claim 44, wherein the first and second enable switchingmeans are external to the integrated circuit.
 46. The apparatus of claim44, wherein the first and second enable switching means are internal tothe integrated circuit.
 47. A method of filtering a phase error signalin a phase lock loop, comprising the steps of: receiving a phase errorsignal from a phase comparator; filtering the error signal with a firstfilter subcircuit when the first filter subcircuit is enabled; andfiltering the error signal with a second filter subcircuit when thesecond filter subcircuit is enabled; and controlling filtering of theerror signal by activating a first enable switch and a second enableswitch in combination, wherein the first filter subcircuit is enabled byactivating a first combination of the first and second enable switchesand the second filter subcircuit is enabled by activating a secondcombination of the first and second enable switches.
 48. The method ofclaim 47, further comprising momentarily activating the second filtersubcircuit to fast acquisition with the first filter subcircuit whereinthe second filter subcircuit has a higher bandwidth than the firstfilter subcircuit.
 49. The method of claim 47, further comprising fastacquisition of one of the first and second filter subcircuits with afast acquisition subcircuit when a fast acquisition switch is activated.50. The method of claim 49, wherein the fast acquisition subcircuitcomprises temporarily enabling the fast acquisition subcircuit tounderdampen the filtering.
 51. The method of claim 49, wherein the fastacquisition subcircuit comprises a fast acquisition resistor which isplaced in parallel connection with a first resistor of one of the firstand second filter subcircuits when the fast acquisition switch isactivated.
 52. The method of claim 51, wherein the fast acquisitionresistor comprises a resistance of approximately R1′ determined by${{R1}^{\prime} = \frac{R1}{\sqrt{F} - 1}},$

where R1 comprises the first resistor of the one of the first and secondfilter subcircuits and F comprises a fast acquisition factor.
 53. Themethod of claim 51, wherein the one of the first and second filtersubcircuits further comprises a second resistor in series connectionwith the first resistor.
 54. The method of claim 53, wherein the fastacquisition resistor comprises a resistance of approximately R1′determined by${{R1}^{\prime} = \frac{{R1}^{\prime\prime}\left\lbrack {R^{\prime\prime} - {{R1}\left( {\sqrt{F} - 1} \right)}} \right\rbrack}{\left( {{R1}^{\prime\prime} + {R1}} \right)\left( {\sqrt{F\quad} - 1} \right)}},$

where R1 comprises the first resistor and R1″ comprises the secondresistor of the one of the first and second filter subcircuits and Fcomprises a fast acquisition factor.
 55. The method of claim 53, whereinthe fast acquisition resistor comprises a resistance of approximatelyR1′ determined by${{R1}^{\prime} = \frac{{R1} + {R1}^{\prime\prime}}{\sqrt{F} - 1}},$

where R1 comprises the first resistor and R1″ comprises the secondresistor of the one of the first and second filter subcircuit means andF comprises a fast acquisition factor.
 56. The method of claim 47,wherein the first filter subcircuit and the second filter subcircuitshare common circuit elements.
 57. The method of claim 56, wherein theshared common circuit elements are a first capacitor and a firstresistor in series both connected in parallel to a second capacitor. 58.The method of claim 56, wherein the first filter subcircuit comprisesfilter circuit elements that are a subset of filter circuit elements ofthe second filter subcircuit.
 59. The method of claim 56, wherein thefirst combination comprises activating the first enable switch anddeactivating the second enable switch which places a first capacitor inseries connection with a first resistor both in parallel connection withsecond capacitor.
 60. The method of claim 56, wherein the secondcombination comprises deactivating the first enable switch andactivating the second enable switch which places a first and thirdcapacitor in parallel connection, both in series connection with a firstresistor and the first and third capacitor and first resistor are allplaced in parallel connection with a second and fourth capacitor inseries connection.
 61. The method of claim 56, wherein the firstcombination comprises activating the both the first enable switch andthe second enable switch which places a first capacitor in seriesconnection with a first resistor both in parallel connection with secondcapacitor.
 62. The method of claim 56, wherein the second combinationcomprises deactivating both the first enable switch and the secondenable switch which places a first and third capacitor and a firstresistor all in series connection, all placed in parallel connectionwith a second and fourth capacitor in series connection.
 63. The methodof claim 47, wherein the first and second enable switches are fieldeffect transistors.
 64. The method of claim 47, wherein when the firstenable switch is activated the first filter subcircuit is alternatelyreferenced to a ground and a common cathode voltage selectable by areference switch.
 65. The method of claim 47, wherein when the secondenable switch is activated the second filter subcircuit is alternatelyreferenced to a ground and a common cathode voltage selectable by areference switch.
 66. The method of claim 47, wherein the first andsecond filter subcircuits each comprise a resistor-capacitor filterincluding a first capacitor and a first resistor in series bothconnected in parallel to a second capacitor.
 67. The method of claim 47,wherein the first and second filter subcircuits are implemented on asingle integrated circuit.
 68. The method of claim 67, wherein the firstand second enable switches are external to the integrated circuit. 69.The method of claim 67, wherein the first and second enable switches areinternal to the integrated circuit.